The object of optical filters, such as the optical filter of the present invention, is to filter light of a selected color, or frequency, and to do so with as narrow a bandwidth as possible. It is known in the prior art of optical filters to provide optical filters wherein layer shaving different characteristics are disposed on plates in order to shift colors. These optical filters are known as interference optical filters. However, interference filters are not very narrow and have very limited applications. They are limited by bandwidth and field of view.
It is also known in the prior art of optical filters to provide birefringent plates wherein the filter characteristics of the plates are dependent upon polarization sensitivity. Because birefringent optical filters use the interference effects of polarization they operate at more narrow specifications than interference filters. However, birefringent optical filters also bottom out and can not provide bandwidth below a known range because transmission drops below a useful level.
In order to obtain more narrow optical filtering than what is provided by interference filters and birefringent filters it is known to use atomic filters. Atomic filters use the characteristics of the atomic structure of selected elements to provide filtering and are therefore able to provide much more narrow specifications. These optical filters only work at certain predetermined wavelengths because there is no continuum in the atomic transition levels giving rise to the filter energy levels. Thus it is desirable to provide atomic filters which are capable of operating at other wavelengths, for example, at the specific wavelength of YAG.
One type of known atomic filter is the Faraday magnetic field rotatable polarization filter. In this type of optical filter a light is applied to the atoms of the filter and the polarization of the light is rotated. If polarization plates at 90.degree. C. are used and rotation around the atomic energy levels is provided these optical filters can be extremely narrow.
The other type of atomic filter known in the art of optical filters is the absorption fluorescent atomic filter. When a photon of an incident light source strikes absorption fluorescent atomic filters it is absorbed by an atom in the filter causing an electron in the atom to rise from one energy level to a higher, or excited, energy level. When the excited electron falls from the higher energy level to an intermediate energy level the energy which is surrendered by the electron is emitted as light energy. The frequency of the -re-emitted light depends on the difference between the excited energy level and the intermediate energy level. A detector senses the re-emitted light. The detector must be able to sense light which is faint with respect to the background light.
It is known to provide two types of absorption filters of this nature, ground state absorption filters and excited state absorption filters. In the ground state absorption filters the electrons which are excited by the incident photons are at their lowest energy level at the time of excitation by the incident photon. In the excited state absorption filters the electrons are pushed to a higher energy level prior to the arrival of the incidentphoton. The process of pushing the electron to the higher energy level is known as pumping. Thus, when the photon imparts energy to the electron in excited state filters the electron rises from one excited state to another, higher, excited state. When the electron falls from the higher excited level to some lower energy level, energy is again emitted as light energy and the sensed by a detector. Thus, several types of optical filters have been developed previously for use in various laser communications and radar systems. Each of these prior art optical filters has its limitations.
Briefly the optical filter of the present invention is based upon the Faraday effect between two excited states in atomic potassium vapor. Zeeman splitting of the atomic energy levels occurs when an external magnetic field is applied to the atomic vapor to cause a differential absorption and dispersion of right-handed and left-handed circularly polarized light. This difference causes plane-polarized light near an absorption transition in the vapor material to be rotated .pi./2 radians with little attenuation. An atomic vapor cell is placed between two cross-polarizers which block all wavelengths of light except those which have been rotated .pi./2 radians. This is the basis for a Faraday rotation optical filter. The Faraday effect for transitions between excited states is caused to occur when the population is pumped into the first excited state from the ground state. In this process the population of electronsis optically excited from the 4S.sub.1/2 ground state into the 4P.sub.1/2 first excited state in potassium using 769.90 nm photons. This excitation requires approximately 10 .mu.J/cm.sup.2 of energy density to saturate the transition and to equalize the population in both the ground state and the first excited state. The excitation excited-state absorption from the 4P.sub.1/2 to the 8S.sub.1/2 levels causes the Faraday rotation at 532.33 nm. The filter is optically pumped with 770 nm light from a Nd:YAG pumped dye laser with a 100 ns pulsewidth and probed with a second dye laser scanned through the 532.33 nm transition.
The optical filter of the present invention offers several advantages over existing filter technology. The measured linewidth of the filter is 0.01 nm and the actual linewidth is believed to be less than 0.01 nm. This linewidth matches the linewidth of the laser used to interrogate the filter. The theoretical linewidth is approximately an order of magnitude less. Furthermore, the actively pumped Faraday optical filter of the present invention may be gated much more easily than other types of filters.